I have started Minecraft with the new Launcher, created a new world, and I started with no Experience bar (As seen in the picture).
Steps to reproduce (works for me):
1. Create a new world in Survival mode.
2. You should have no Experience bar.
I tried starting Minecraft multiple times and it still is invisible.
EDIT: It reappers if you earn a full level.
Duplicate of MC-13592, please use the search function to see if your bug has already been submitted. Currently over 54% of tickets are being closed as duplicate.